ANTICOINCIDENCE RING A ring of counters connected to exclude outside radiation. BACKGROUND COUNT The number of impulses per unit time registered on a counting instrument when no sample is present. BETA DECAY Radioactive decay with emission of a beta particle. BETA PARTICLE An electron emitted by a nucleus. BRACKETED INTRUSIVE Igneous rock extending into sedimentary rocks that are datable by their fossils. CLOSED SYSTEM A system in which the parent material radioactively decays into its daughter products and nothing is added or removed. COMMON (strontium, lead, etc.) The ordinary element present in nature at any one time as distinguished from that produced by radioactive decay. CONCORDIA ANALYSIS A mathematical technique to determine graphically the age of a material containing radiogenic lead by comparing its uranium-to-lead ratio with the similar ratio in a closed uranium-lead system. CONTACT METAMORPHISM A metamorphism genetically related to the intrusion of molten masses of rock and taking place at or near the contact. COSMIC RAYS High-energy particles moving in our galaxy. CRYSTAL A periodic or regularly repeating arrangement of atoms, formed from a single element or compound. DAUGHTER A nuclide formed from the radioactive decay of another nuclide. DECAY CONSTANT The number of atoms decaying per atom per unit of time (0.693/half-life). ELECTRON CAPTURE A nuclear process in which the nucleus of an atom captures an electron from one of the inner shells. ELECTRONS Elementary particles with a unit negative electrical charge and a mass 1/1837 that of the proton, or 9.12 × 10⁻²⁷ gram. Electrons surround the atom’s positively charged nucleus and determine the atom’s chemical properties. GAMMA RAYS Electromagnetic radiation from an atomic nucleus. GEIGER COUNTERS Instruments that count pulses produced by radioactivity, consisting of a counting tube with a central wire anode, usually filled with a mixture of argon and organic vapor. HALF-LIFE The time it takes for half the atoms in a radioactive substance to decay. ION An atom or molecule that has lost or gained one or more electrons and is thus electrically charged. ISOTOPE DILUTION An analytical technique involving addition of a known amount of an isotopic mixture of abnormal composition to the unknown amount of an element of normal or known isotopic composition. ISOTOPES Nuclides of the same atomic number but different atomic weight. Isotopes of a given element have an identical number of protons but different numbers of neutrons in their nuclei. LAW OF SUPERPOSITION Statement that overlying strata must be younger than underlying strata if there has been no inversion. MASS SPECTROMETER An instrument for separation and measurement of isotopes by their mass. NET COUNTING RATE Sample counting rate minus background counting rate.